feat(platform/bitbucket): add support for fetching changelog/release-notes

[setchy]

Type of discussion.

I'm proposing an idea

Tell us more.

I'd like to suggest an enhancement for platform:bitbucket which will add support for fetching changelogs.

Implementation-wise, looks like it would roughly involve mirroring the github / gitlab functionality within https://github.com/renovatebot/renovate/tree/main/lib/workers/repository/update/pr/changelog

[setchy]

I found #14964. Will move the discussion back into that ticket